Mesotheliomas are rare but increasingly important malignant neoplasms which occur almost exclusively in persons exposed to asbestos. The lesions are difficult to diagnose and universally fatal. Our understanding of the pathogenesis of mesotheliomas is limited. Although animal models exist, they have not been studied comprehensively from the perspective of the biology of the tumor and the factors that play a role in neoplastic transformation of mesothelial cells. In this investigation, we will study tumors induced in rats with crocidolite to evaluate in detail the evolution and location of the asbestos-induced lesions in the pleura of the lung. The lesions will studied histologically and cytologically, using a variety of morphologic techniques. Cells in preneoplastic and neoplastic lesions will be evaluated and attempts made to determine their biologic properties by appropriate in vitro techniques and transplantation into syngeneic animals. This investigation will provide an in-depth insight into the mechanisms of neoplastic transformation of the mesothelium and attempt to explain how asbestos plays a pathogenetic role in this neoplastic process.